1. Field of the Invention
The present invention relates to an optical apparatus having a moving lens.
2. Related Background Art
In the field of the optical apparatus including cameras, compactification of photographing optical systems and reduction of image size of solid state image sensing devices have quickly been advancing these years. In addition, plastic materials are often used as optical materials for making the photographing optical system. Use of plastic materials has such features that a lens can be formed readily by mold, that arbitrariness of shape thereof is high, and that the cost merit is higher than that of glass materials. Because of such features, lenses made of plastic materials are frequently used in a viewfinder system, in an infrared active autofocus unit, in parts of the photographing optical system, and so on.
The plastic materials show greater changes of physical properties against environmental changes than inorganic glass materials. For example, PMMA of a plastic material has a large coefficient of linear expansion: 67.9.times.10.sup.-6 /.degree. C. typical, whereas LaK 14 (available from OHARA) of an inorganic glass has a coefficient of linear expansion one order of magnitude smaller than it: 57.times.10.sup.-7 /.degree. C. As for changes of refractive index against temperature changes, PMMA shows 1.0 to 1.2.times.10.sup.-4 /.degree. C. typical, whereas the above LaK 14 shows 3.9 to 4.4.times.10.sup.-6 /.degree. C at the D-line two orders of magnitude smaller than those.
As explained above, the plastic materials show greater changes of various optical constants (the refractive index, the shape, etc.) against temperature changes than the inorganic glass materials. For example, the lenses made of the plastic materials, so-called plastic lenses, have greater changes of focal length against temperature changes than the lenses made of the inorganic glass materials.
Further, the plastic materials have larger water absorption rates than the inorganic glass materials. Therefore, the various optical constants of the plastic lenses change greater against humidity changes, similarly as against temperature changes, than the lenses made of the inorganic glass materials.
The effects as discussed previously can be attained by using the plastic lens in parts of the optical system. However, it raises the problem that the optical properties including the focal length change greater against environmental changes, particularly against temperature changes or against humidity changes, than in the case of use of the lenses made of the inorganic glass materials.
The recent optical apparatus is compactified by compactifying the photographing optical system, compactifying the solid state image sensing device, and increasing the density of various elements. This raises the problem that the temperature changes, moisture changes, or the like increase an effect of deviation of the image plane of the optical system used in the optical apparatus with respect to the intended image plane. It is, therefore, a significant issue how to effectively correct the deviation of the imaging position due to such environmental changes.
Further, many recent optical devices are provided with an autofocusing function (AF function) for automatically detecting the in-focus position of the optical system. During photography with operation of the AF function, for example, if any obstacle such as a car or a pedestrian goes between the optical apparatus and the object, the AF means will make the optical system in focus with the obstacle. This will result in failing to image the target object on the image plane. Thus, there was the problem that photography must be made under stop of the AF function in order to avoid it. Stop of the operation of the AF function during photography is not preferred, because the position of the image plane changes greatly with the environmental changes as discussed previously in the case of the optical apparatus using the optical system including the plastic lens.